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1. Evolution of Surface Chemistry in Two‐Dimensional MXenes: From Mixed to Tunable Uniform Terminations

   https://doi.org/10.1002/anie.202409480  

   Jiang, Mengni; Wang, Di; Kim, Young‐Hwan; Duan, Chunying; Talapin, Dmitri_V; Zhou, Chenkun (August 2024, Angewandte Chemie International Edition)

   Abstract Surface chemistry of MXenes is of great interest as the terminations can define the intrinsic properties of this family of materials. The diverse and tunable terminations also distinguish MXenes from many other 2D materials. Conventional fluoride‐containing reagents etching approaches resulted in MXenes with mixed fluoro‐, oxo‐, and hydroxyl surface groups. The relatively strong chemical bonding of MXenes’ surface metal atoms with oxygen and fluorine makes post‐synthetic covalent surface modifications of such MXenes unfavorable. In this minireview, we focus on the recent advances in MXenes with uniform surface terminations. Unconventional methods, including Lewis acidic molten salt etching (LAMS) and bottom‐up direct synthesis, have been proven successful in producing halide‐terminated MXenes. These synthetic strategies have opened new possibilities for MXenes because weaker surface chemical bonds in halide‐terminated MXenes facilitate post‐synthetic covalent surface modifications. Both computational and experimental results on surface termination‐dependent properties are summarized and discussed. Finally, we offer our perspective on the opportunities and challenges in this exciting research field. 

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2. Computationally Guided Synthesis of MXenes by Dry Selective Extraction

   https://doi.org/10.1002/adma.202305200  

   Rems, Ervin; Anayee, Mark; Fajardo, Eiara; Lord, Robert_L; Bugallo, David; Gogotsi, Yury; Hu, Yong‐Jie (October 2023, Advanced Materials)

   Abstract MXenes are a rapidly growing family of 2D transition metal carbides and nitrides that are promising for various applications, including energy storage and conversion, electronics, and healthcare. Hydrofluoric‐acid‐based etchants are typically used for large‐scale and high‐throughput synthesis of MXenes, which also leads to a mixture of surface terminations that impede MXene properties. Herein, a computational thermodynamic model with experimental validation is presented to explore the feasibility of fluorine‐free synthesis of MXenes with uniform surface terminations by dry selective extraction (DSE) from precursor MAX phases using iodine vapors. A range of MXenes and respective precursor compositions are systematically screened using first‐principles calculations to find candidates with high phase stability and low etching energy. A thermodynamic model based on the “CALculation of PHAse Diagrams” (CALPHAD) approach is further demonstrated, using Ti₃C₂I₂as an example, to assess the Gibbs free energy of the DSE reaction and the state of the byproducts as a function of temperature and pressure. Based on the assessment, the optimal synthesis temperature and vapor pressure are predicted and further verified by experiments. This work opens an avenue for scalable, fluorine‐free dry synthesis of MXenes with compositions and surface chemistries that are not accessible using wet chemical etching. 

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3. High-throughput density functional theory screening of double transition metal MXene precursors

   https://doi.org/10.1038/s41597-023-02755-2  

   Nykiel, Kat; Strachan, Alejandro (November 2023, Scientific Data)

   Abstract MXenes are an emerging class of 2D materials of interest in applications ranging from energy storage to electromagnetic shielding. MXenes are synthesized by selective etching of layered bulk MAX phases into sheets of 2D MXenes. Their chemical tunability has been significantly expanded with the successful synthesis of double transition metal MXenes. While knowledge of the structure and energetics of double transition metal MAX phases is critical to designing and optimizing new MXenes, only a small subset of these materials been explored. We present a comprehensive dataset of key properties of MAX phases obtained using density functional theory within the generalized gradient approximation exchange-correlation functionals. Energetics and structure of 8,712 MAX phases have been calculated and stored in a queryable, open database hosted at nanoHUB. 

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4. Vanadium MXenes materials for next-generation energy storage devices

   https://doi.org/10.1088/1361-6528/acc539  

   Sijuade, Ayomide Adeola; Eze, Vincent Obiozo; Arnett, Natalie Y; Okoli, Okenwa I (April 2023, Nanotechnology)

   Abstract Batteries and supercapacitors have emerged as promising candidates for next-generation energy storage technologies. The rapid development of new two-dimensional (2D) electrode materials indicates a new era in energy storage devices. MXenes are a new type of layered 2D transition metal carbides, nitrides, or carbonitrides that have drawn much attention because of their excellent electrical conductivity, electrochemical and hydrophilic properties, large surface area, and attractive topological structure. This review focuses on various synthesis methods to prepare vanadium carbide MXenes with and without etchants like hydrofluoric acid, lithium fluoride, and hydrochloric acid to remove the ‘A’ layers of the MAX phase. The goal is to demonstrate the utilization of a less toxic etching method to achieve MXenes of comparable properties to those prepared by traditional methods. The influence of intercalation on the effect of high interlayer spacing between the MXene layers and the performance of MXenes as supercapacitor and battery electrodes is also addressed in this review. Lastly, the gaps in the current knowledge for vanadium carbide MXenes in synthesis, scalability, and utilization in more energy storage devices were discussed. 

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5. Topochemical synthesis of 2D materials

   https://doi.org/10.1039/c8cs00649k  

   Xiao, Xu; Wang, Hao; Urbankowski, Patrick; Gogotsi, Yury (November 2018, Chemical Society Reviews)

   Since the demonstration of the unique properties of single-layer graphene and transition metal dichalcogenides (TMDs), research on two-dimensional (2D) materials has become one of the hottest topics, with the family of 2D materials quickly expanding. This expansion is mainly attributable to the development of new synthesis methods to create new materials. This review will summarize and critically analyze topochemical synthesis methods for synthesizing novel 2D materials. For example, the emerging family of 2D transition metal carbides, nitrides and carbonitrides (MXenes) are synthesized primarily by selective etching of “A” (metal) elements from MAX phases. Another 2D material, hydrogenated germanene is produced by selective etching of calcium digermanide (CaGe 2 ). The topochemical transformation of one dichalcogenide into another and 2D oxides into 2D carbides or nitrides have attracted great attention because materials with many useful and diverse properties can be obtained by these methods. Topochemical synthesis methods provide alternative ways of synthesizing 2D materials not requiring van der Waals bonded solid precursors or vapor phase deposition, but they have not been comprehensively reviewed. In this review, we describe common principles of topochemical synthesis of 2D materials, explain synthesis mechanisms and offer an outlook for future research. 

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